Chlamydia trachomatis is a gram-negative bacterial pathogen that produces a host of major sexually transmitted diseases that result in serious reproductive morbidity. The organism is an obligate intracellular parasite with a complex and poorly understood development cycle involving the serial alternation of two morphologic forms. The goal of this project is to understand the genetic and molecular basis of this developmental cycle. We and others have previously characterized the cis acting signals involved in chlamydial gene expression. We now propose (i) to characterize the chlamydial RNA polymerase and to set up systems for the expression of chlamydial genes in vitro or in E. coli; (ii) to use these systems to further define chlamydial expression signals and the factors that regulate them; (iii) to identify developmentally regulated genes involved in the terminal stages of chlamydial differentiation during infection; and (iv) to develop methods for gene transfer into chlamydiae, to allow genetic dissection of the roles of chlamydial genes in the life cycle.